#include <bits/stdc++.h>
#ifdef BUG
    #include "debug.hpp"
#else
    #define DEBUG(var)
#endif

using namespace std;
template< class T1, class T2 > inline istream &
operator>>( istream & fin, pair< T1, T2 > & pr )
{ fin >> pr.first >> pr.second; return fin; }
template< class T0, class T1, class T2 > inline istream &
operator>>( istream & fin, tuple< T0, T1, T2 > & t )
{ fin >> get<0>(t) >> get<1>(t) >> get<2>(t); return fin; }
template< class T > inline istream &
operator>>( istream & fin, vector< T > & a ) {
for(auto & u: a) fin >> u; return fin; }
template<class T, size_t n> inline istream &
operator>>( istream & fin, array<T, n> & a ) {
for(auto & u: a) fin >> u; return fin; }
template<class FwdIter> inline auto
dump(FwdIter first, FwdIter last, const char * dlm = " ") -> void {
typedef typename iterator_traits<FwdIter>::value_type value_type;
copy(first, last, ostream_iterator<value_type>(cout, dlm)); }
template<typename T> vector<T> & operator--(vector<T> & a)
{ for(auto & i: a) --i; return a; }
/* @@@ ----------------------------------- */

int64_t turn_off_the_lights()
{
    size_t n, k;
    cin >> n >> k;
    vector<int64_t> c(n);
    cin >> c;

    const int64_t inf = numeric_limits<int64_t>::max() >> 2;
    vector<int64_t> left(n, inf), right(n, inf);

    for(size_t i = 0; i < n; ++i)
    {
        const size_t l = i < k ? 0 : i - k;
        const size_t r = min(i + k, n - 1);
        left[r] = min(left[r], c[i] + (l ? left[l - 1] : 0));
    }

    for(size_t i = n - 1; i < n; --i)
    {
        const size_t l = i < k ? 0 : i - k;
        const size_t r = min(i + k, n - 1);
        right[l] = min(right[l], c[i] + (r != n - 1 ? right[r + 1] : 0));
    }

    DEBUG(left);
    DEBUG(right);

    int64_t out = min(left.back(), right.front());
    for(size_t i = 0; i + 1 < n; ++ i)
        out = min(out, left[i] + right[i + 1]);

    return out;
}

int main(const int argc, char * argv [])
{
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    cout << turn_off_the_lights();

    return EXIT_SUCCESS;
}